The present invention relates generally to rolling mills and more particularly to a method and apparatus for providing a more accurate signal, representing the actual roll separation force occasioned by the presence of a workpiece between the rolls of the mill, for use with an automatic gage control system.
One well known method of controlling workpiece gage is that which is commonly referred to as the BISRA gagemeter automatic gage control (AGC) system. In this system the force associated with and generated by the workpiece as it passes through the stand workrolls is sensed and combined with a signal proportional to roll position to form a signal representative of workpiece thickness which is used in a closed loop system to adjust the gap or opening between the opposed workrolls. In applications where incoming workpiece hardness and thickness variations are less significant than mill roll irregularities, such as eccentricity or ovalness, the thickness control strategy may be used for regulation of rolling force on the assumption that constant rolling force will produce uniform output thickness.
There are at least two well known methods of sensing this force. The first of these methods is what is here termed the direct method and commonly uses load cells placed between the mill housing and the roll gap to provide an output force signal. An alternative to using the load cells, where such is used, is to sense the pressure within a hydraulic cylinder which is used as a gap adjusting means in the automatic gage control system. The second method which is here termed an indirect method uses strain gages, located on the mill housing, to measure the strains on that housing when a workpiece is being rolled.
In practice neither of these systems has been as accurate as might be anticipated. One of the prime causes of inaccuracies in the direct method is friction. As is well known in the art, friction exists between the mill stand housing and the chocks which support the rolls as well as in certain hydraulic elements such as balancing jacks which are used to maintain the roll chocks in position and, where used, the hydraulic roll gap adjustment mechanism. Since both gagemeter and force control systems employ the use of a force feedback signal, it is apparent that any forces seen by the force sensor in addition to those forces produced by reduction of the workpiece will tend to degrade the accuracy of that force signal as a true representation of the actual rolling force. It must be remembered that in all gage control systems the gap between the rolls is repeatedly being changed in an attempt to effect constant output gage as a function of the force feedback signal.
It is also recognized in the art that the frictional forces are not constant but vary in accordance with mill conditions and the direction of roll travel as the roll gap being adjusted. These produce what in effect is commonly referred to as a hystereis. A more complete discussion of the frictional forces and the hystereis effect may be had by reference to the following two articles:
(a) "Mill modulus variations in hystereis--Their affect on hot mill AGC" by G. E. Wood et al., Iron and Steel Engineer Yearbook, 1977, pages 33 through 39; and,
(b) "Force sensing in rolling mill" by A. Zelpkalns et al., Iron and Steel Engineer Yearbook, 1977, pages 40 through 46.
The strain gage method of producing the force signal is far less susceptible to friction forces than the direct method just discussed but is highly susceptible to temperature. That is, the strain gage method does not see chock-housing frictions, which are normally the largest components of friction, although it is somewhat susceptible to friction of the gap adjusting cylinder as well as balance jack cylinder friction when the workroll balance jacks are between the workroll chocks and not abutting the housing. Temperature, on the other hand, plays a significant factor in the output of the strain gage system and in order to make this system practical, the strain gage must be continuously calibrated for temperature. This is not practical in many instances, particularly when the rolling mill is continuous rather than reversing and the time between unloaded states may be several minutes.